Use 'slash' on input files before globbing to support absolute paths with backslashes on Windows

[matthias-christen]

globby (used to glob input arguments to postcss-cli) only supports forward slashes.
In order to be able to provide absolute paths containing backslashes to the CLI on Windows, they need to be converted to forward slashes, otherwise postcss-cli terminates with the error message 'Input Error: You must pass a valid list of files to parse'.

[coveralls]

Coverage increased (+0.1%) to 82.237% when pulling a0d4fb0 on matthias-christen:master into c94c188 on postcss:master.

[RyanZim]

Forgive me for my ignorance (I've never actually run postcss on Windows), but why can't you just pass paths with forward slashes from the CLI?

[matthias-christen]

@RyanZim, on Windows, the natural directory separator is a backslash. For instance, if you tab-autocomplete a path in a Windows command line, it will convert forward slashes to backslashes. So, in particular for a command line tool, I think it is important to accept paths in the system's standard format.
What is really confusing currently: if you supply a (seemingly) perfectly valid absolute path to postcss, you'll get the error message "Input Error: You must pass a valid list of files to parse". I guess you could also adjust the error message that the user must enter forward slashes, but why not just do it right when the fix is so simple.

[RyanZim]

Got it, tab-autocomplete. That makes sense; LGTM.

[RyanZim]

Will try to release this later this week.

[RyanZim]

Eh, that was a bit longer than a week. Regardless, published in v8.2.0. 🚀